Ultra high frequency attenuator



ZAQLGQ Dec. 20, 1949 H, Rf LARSON ULTRA lHIGH FREQUENCY ATTENATOR 3 Sheets-Sheet 1.

Filed April 1e,V 1945 Dec. 20, 1949 H. R. LARSON 2,491,669

ULTRA HIGH FREQUENCY ATTEQIUATOR Filed April, 16, 19454: 5 Skneecs-Sheerl 2 l 90 Q \9 s. \1- q (ATTO/M057 Dec. 20, 1949 l-a. R. LARSON 2,491,669 ULTRA HIGH- 'FREQUENCY ATTENUATOR FiledApril 1e, 1945 s sheets-sheet 3 Patented Dec. 20, 1949 UNITED STATES PATENT GFFICE 214911569 `til/rita HIGH FREQUENCY ATTENTO' Harry i Jarson,QlL-1 icago..lul,; assigner to. West,-

ernrElectric Company, Incorporated, New York, N. Y., a corporation of New York Application April 1e, 192i5', serial No; 588,702

2` claims. (ci. 1re-44') This invention `relates' to an apparatus for measuring electrical properties, and more particularly to a radio frequency attenuator.

Three important reasons dictate the necessity for precise impedance matching in ultra-high frequency systems: Power transfer is at a' maximum when impedances are matched. Mismatched impedances' in' an .ultra-high frequenc system will cause undesirable reflections i the waves traveling through `the component parts. Reenforcement of the incident waves by these reilections may cause dielectric breakdown in the system. Reflections will also causeu frequency drift and may cause the oscillator tube to completely cease functioning; It is well known that the standing wave' ratio in an ultra-high irequency system is a `function' of the impedance relationships in' the' system and` that a ratio higher than unity indicates an' impedance mismatch.

The ei'istence of mismatched impedances in ultra-high frequency systems' is ordinarily ascertained from standing wave observations made with what is `known in' the art as `a traveling standing wave detector inserted into the line and electrically connected toa current indicating device, usually a micro-amineter,` `t`o..indicate. the rectified output of the detector. The rectiiying element of `the detector is in most cases asilicon crystal. Investigation has proved that the response of the'I crystal does' notA follow a square law nor a different butcoristantpower (exponent) law. With rare exceptions, the crystal response follows a widely variable 'power law as the load on the crystal changes through the ranges involved in standing wave measurements. Neither a square law assumption nor a calibration based `on a" constant exponent law'for the crystal will give the precise standing wave ratio measurements so urgently necessary in the rened micro-Wave' techniques of today,iespeci'ally in radar.

Even if 'a careful and laborious point-by-point calibration of the crystal' is made,` it may vcharige in unpredictable `direction' and magnitude from day to' day as atresult ofcomparatively slight changes in' conditions under which `it .is used. Frequently changes take place for 4Which no assignable cause has' beenfound. It is'obvious that a' methodof'standing wave measurement which is independent of 'the crystal characteristics is not only desirable but essential if precise measurements are' required. i

Objects of this invention are to provide anapparatu's for very precisemeasurement of electrical characteristics;

In' accordance with one embodiment of this YI? invention a continuously variable radio frequency attenuator, with a micrometric depth adjustment and a depth gage, calibrated directly in standing wave ratios or decibels,` is inserted into a longitudina'l slot cut into' a wave guide section. The attenuatingel'em'ent is a rectangular piece of resistive strip half a Wave length long, or if a higher degree of attenuation is desired its length may be an integral number of half-wave lengths. This length is selected to avoid phase eiects on the attenuation'.

The attehuator is inserted in a system Vbetween an ultrahigh frequency generator and a traveling standing wave detector which is connected to the unknown section .under test which in turn is coupled to a totally absorbing termination. The probe of the detector is movedA until themin- `irnum currentvreading is indicated' on the detector meter, then the probe is .moved to the point `or highest current." Sufficient attenuation is then "introduced to' bring the detector meter needle `back" to the minimum' current value lrst noted. The calibrated attenuator then yields the value of the standing wave ratio directly.`

A complete understanding of the invention may 'be had by reference to' the' 'following description taken in conjunction with the accompanying drawings; in which Fig. 1 is a plan view of an apparatus embodying' thev invention and showing ai method of connecting the` unknown section in the line;

Fig. 2v is a' front' elevation of the apparatus shownin' Fig'. 1 with part in'section;

Fig. 3 is a cross-sectional view of the attenuator taken on the' line 3-3 ofFig. 2;

Fig.' 4 is a' Vertical'section of the detector taken 4on line 4-4' of Fig.' 2;

Fig'. 5 is a' vertical section oi the attenuator taken on line 545 of Fig. 2 with a fragmentary elevational view of themeter;

Fig.' '6 is a central' longitudinal sectional `view of the attenuator taken on the line 0 5 of Fig. 5;

Fig. 'l is a horizontal sectional View of the attenuator taken on the line 1-1 of Fig. 6;

Fig. 8 is a plan View of a modiiied form of the apparatus' showing the attenuator inserted between the probe and' the crystal of the detector; Fig. 9 i's a sectional view of the apparatus shown in Fig. 8 taken on the line A9 9 of Fig. 8 with .part shown in elevation, and

Fig. `10 is a graphic illustration showing the relationship of the observed current curve to the cos2curve."

The apparatus/ embodying the invention com- :plises an ultra-high frequency source of energy lintegral number of half wave lengths. 'tenuating element, a rectangular piece of 200 Aabsorbing ultra-high frequency U such as a magnetron oscillator or a velocity modulated tube generator, a movable detector for measuring the energy at different points along a wave guide, and an attenuator by means of which the energy in the wave guide may be attenuated so that the detector when set at the maximum point of a standing wave may be made to indicate the same value as at a minimum point and the relative value of reflected energy may then be determined by the amount of attenuation.

A radio frequency attenuator indicated generally at 9 is built upon a section of rectangular wave guide Ill, which is provided with suitable coupling flanges il and I2 to permit the conneo- 1 tion of the section into an ultra-high radio frequency system. Transverse leakage of power at the joints is prevented by a circular groove I3 formed into the face of the flange which acts as a radio frequency choke.

wave guide and retracted therefrom at will. If a higher degree of attenuation is desired, the l length of the element I5 may be increased an The atohms per square inch resistive strip capable of clamped with the plane of the strip parallel to the length of the wave guide, between a block I6 and a cross bar II having extended lugs 2l and 22 slidably engaging two vertical guide slots 23 and 24 formed in the legs of a main support member 25 having an inverted U-shaped lower portion which is firmly mounted on two rectangular uprights 26 and 21 secured to the wave guide I0 at either end of the slot I4.

Vertical reciprocation is imparted to the attenuating element by fastening the cross bar I'I to the lower end of a vertically disposed rod 3l reciprocably supported in a bore 32 formed in the main support member 25, the lower end of the bore terminating in an aperture 33 which is reduced in diameter to guide' the downwardly extending lower end of the rod. The upper end of the rod is threaded to cooperate with a thumb nut 34 which, when manually turned, will impart a gradual vertical movement to the rod either up or down, depending on the rotational direction of the thumb nut. An annular shoulder 35 on the rod 3i slidably engages the walls of the bore 32 to firmly guide the rod. The portion of the rod extending downward from the shoulder 35 ,1

is reduced in diameter to provide room between the wall of the bore and the rod for a helical 'compression spring 36 which encircles the rod between the shoulder 35 and the lower end of the bore. This spring exerts a constant upward pressure against the shoulder 35 which pressure is transmitted through the rod 3l to the thumb -nut 34, the upper part of which is urged against the lower side of the horizontal portion of an inverted U-shaped gage clamping fixture 3l, maintaining firmness of control and preventing backlash. Lateral displacement of the clamping xture is prevented by tting the lower part of the legs of the clamp into vertical grooves 5I and 42 formed in the main support '25 to which the clamp is secured. To limit the vertical position of the gage clamp 31, the legs of the clamp have shoulders 43 and 44 formed thereon to abut the top of the main support 25. Space between the top of the main support 25 and the yoke of the A longitudinal slot I4 of proper size is cut into one of the wide sides `of the guide to allow an attenuating element I5, `a half wave length long, to be inserted into the 4i clamp is provided to accommodate the thumb nut 34.

Clamping pressure applied laterally by two screws 45 and 46 grips the downwardly extending bushing 49 of a dial type gage 50 in the circular enlargement of a slot 5I cut lengthwise through the yoke of the clamp 3`I, the opposite faces of the slot operating as a vise. A'n actuating plunger 52 on the gage 50 passing through the bushing 49 bears against the flat top of the rod 3l so that the gage will respond to and indicate the vertical position of the rod 3l with reference to a zero point. It is apparent that operating the thumb nut 34 will impart vertical displacement to the rod which in turn will either lower the attenuating element I5 into the wave guide I3 through the slot I4 or retract it depending upon the direction of rotation of the thumb nut. It is further evident that the depth to which the attenuating element is lowered into the wave guide IU will be indicated on the gage 50.

The degree of attenuation introduced is a function of the depth that the attenuating element reaches in the wave guide. The ratio between two power levels of standing waves is made a function of the degree of attenuation that is introduced to reduce the upper power level to the value of the lower. This permits the device indicating the depth of the attenuating element (in this case the dial type gage) to be calibrated directly in standing Wave ratios or in decibels.

The attenuator 9 may be calibrated by coupling it between an ultra-high frequency gener ator 53, such as a magnetron oscillator, a velocity modulated tube generator, or the like, and an ultra-high frequency detector indicated generally at 54 and described later herein. For the purpose of calibration, it is not necessary that the probe of the detector be horizontally moved, because during the calibration standing waves are not present in the apparatus and the power level at any given instant is the same through the wave guides of the apparatus. The load characteristic of the crystal rectier in the detector used for Calibrating the attenuator should be accurately determined beforehand.

Since the crystal response does notfollow the square law of power, its response to the various power levels must be corrected for each observation to give a true indication of the power level observed. The load characteristic of the crystal may be determined by making a traverse of approximately a wave length at the frequency at which the crystal is to be used with a standing wave detector in a line terminated with a short circuit so that the minima will be zero. The response of the crystal is observed at intervals of l millimeter and a curve 58 is plotted with the current readings representing the crystal response as ordinates and the millimetric readings of the detector position as the abscissae. The linear measure of the detector position is convertedinto the electrical angle at each point using the maximum point as the origin as indicated in Fig. 10. A. cos2 curve 59 having the same origin and maximum amplitude is plotted using the maximum observed current times cos20 as the ordinates and the 0 as the abscissae, 0 being the electrical angle. The corrected response of the crystal for any given angle will be the ordinate of the cos curve at that angle.

A totally absorbing termination 55, hereinafter described, is coupled to the other end of the standing wave detector 54.. A micro-ammeter 51 aesinet is connected. to the detector 54; to measure the rectified output of the crystal. Corresponding readings of current and attenuator settings are .made-through the range of the attenuator. The

current values are then corrected in accordance with the` known characteristics ofthe crystal and the standing wave ratios or the decibels corresponding to` the readings are computed and may be marked directly on the dial; of the gage 5D.

rI The calibration of the attenuator will be valid indefinitely, and will thereafter be unaffected by any changes` in the crystal loadl characteristic or by the substitution of another crystal with dif'- ferent load characteristics.

To determine the impedance of an unknown section d of an ultra-high frequency system, the output of the radio frequency generator 53; is

coupled to; the input of the calibrated radio fregquency` attenuator e through a suitable transmission lineor wave guide 61|. The output of the attenuator is coupled to one endv of a traveling standing wave detector shown generally at 5d which preferably should comprise a section of wave guide Se with coupling flanges G5 and 65 at both ends, and providedwith a slot 61 into whichY the end of a traveling probe 1| extends. ixedto the wave guide 64 is a bed block 1.2 having a slot '1.3 therein which` coincides with the slot G2. Ai, carriage block I4 rides upon a pair of.' ways l5 and 'L6 secured to the bed block 'I2 which also supports a pair of racks 8| and 82,

which in` combination withl pinion gears 83 and 8e actuated by a knurled knob 8'5 provide variable horizontal movement for the carriage block 14.

Fitted into a circular aperture 86, of the carriage block is a bushing 8'! which houses a probe insulating bushing B8. The bushing 81 opens upwardly into a pickup wave guide 9| disposed at right angles to the wave guide 64, and secured to the carriage block 1'4.

A.` housing 92 for supporting the mechanism for vertically adjusting a probe shaft 93, to which the probe 'H is fixed, is mounted on the top, of the pickupwave guideldirectly` over a circular aperture dit formed in the wave guide 9|. Penetration of the probe 'II into the wave guide (Mis controlled by a knurled nut 95,. Another knurled nut 91df controls a coaxial'tuner 91 to match the probe to the wave guide 9|.. An` independently mounted tunable crystal I! is located in the pickup guide 9| approximately a half wave length from a fixed back plate 02. The crystal is tuned by means of a knurled head |03 on the adjustable screw ldd mounted in a housing attached to the pickup guide 9|. The output of the crystal is connected to a micro-ammeter 5T through a coaxial cable |51 and a coaxial terminal |08. The other end of the detector 54 is coupled to one end of the unknown section 6!! under test, which may be a section of wave guide, a junction box or any part of a system, the impedance of which is to be observed. The unknown section is coupled at its other end to the totally absorbing termination indicated generally at 5B and which comprises a short length of flanged wave guide I in which a triangular piece of resistive strip ||2 is secured to the end of a reciprocably adjustable rod I I s which passes through an insulating bushing H4.

Electro-magnetic waves produced by the oscillations of the radio frequency generator 53 are transmitted through the wave guides of the various components, passing through the attenuator guide section I3 first, then through the guide section 54 of the detector 54, then through the unknown section 60;, finally striking the termina.- tion 56.

If the impedance of the` unknown section matches that of the system, maximum power transfer will be effected and no reflection will take place, but if the impedance of the unknown section does not match` the; system impedance, .then reiiections of the original waves 'will be produced. The amount of reiiectionis` dependent .upon the degree of mismatch between the im.- ;pedance of the unknown section and that of the system. These reiiections, cause standing waves to be set up in the wave guides. The detector probe il is moved` horizontally by means of the knob 8,5. along the horizontal slot 61 until the minimum current indicated on the micro-ammeter el in the detector circuit andthis value is noted. Next, the detector probe '|,I iszagain moved 'ai-ong the slot until the` maximum current is shown on the meter 5.1.v Attenuation is then provided by turning the thumb screw 3ft of the attenuator S whichf willV cause the attenuating element i5 to enter the waveguide Ill through. the slot ifi. Sufcient attenuation is introduced. to bring the needle of the meter 51 back to the mini.- rnum reading` first noted. The calibrated dial gage on the attenuator 9 will.l then indicate the standing wave ratio..

The standing wave, ratio isl thus directly observed. Laborious and time-consuming calculations are unnecessary, and. the results are extremely accuratebecausethey are independent of the shape of the crystal characteristic. The crystalL follows yessentiallyl thesame pattern for both theascending,andfdescending branchesof its load characteristic. The consequences of inconsistencies of the current indicating meter are also eliminated since both initial and finalv readings are made atthe. samepoint of themeter scale.

This radio frequency attenuator is phase insensitive due to the attenuating element` i5V being a half wave length or an` integral number of half- Wave lengths thusfenabling it to intercept the equivalent cfa full loop or loops of the standing Wave regardless of its position inthe guide with respect. to rthe standing waves.

Although the methodr described is for use in zlowpower systems, itz-canbe successfully used in high. power systems by inserting the radio frequency attenuator between the probe of the standing wave detector and the rectifier element of the standing wave detector. Between these points the power is relatively low and the attenuating element will not burn up. An embodiment of this modication of the invention herein is disclosed in Figs. 8 and 9 wherein the output of a source |2| of ultra-high frequency oscillations, which may be a magnetron oscillator or a velocity modulated tube generator, is connected to one end of a standing wave detector, shown generally at |22, which comprises all the elements of the one indicated at 54. In addition, however, the upper surface of the pickup wave guide I 2s of the standing wave detector |22 is provided with a pair of uprights |24 and |25 to support a radio frequency attenuator indicated generally at I 2G and which is of the same construction as the radio frequency attenuator indicated at 9, except that the pickup wave guide |23 is substituted for the wave guide IIJ of the formerly described attenuator. A slot |3I is formed in the pickup guide |23 to receive an attenuating element |32 which may be inserted into and retracted from the guide |23 by means of a continuously variable vertical adjustment controlled by thumb nut |33, the detail of this adjustment being the same as that of the radio frequency attenuator indicated at 9. The length of the attenuating element is one-half wave length to prevent phase sensitivity. A microammeter |34 is connected to the output of the crystal rectier of the standing wave detector |22 to measure the value of the rectified current. The other end of the detector |22 is coupled to one end of an unknown section |35 to be tested, the other end of which is coupled to a totally absorbing termination |36 which is in all respects the same as the one shown generally at I |2.

This method of impedance testing is the same as that described in the first embodiment of this invention. Electromagnetic waves produced by the radio frequency generator 12| pass through the Wave guides of the detector |22 and the unknown section |35, striking the termination |35. If the unknown section |35 possesses a characteristic impedance that permits unopposed passage of all the power entering it, then the incident Waves striking the termination |36 will be absorbed by the termination; however, should the unknown section oier opposition to maximum power transference, then the incident waves will be reflected causing standing waves to be produced in the wave guides of the apparatus.

The probe 14| of the detector |22 is moved horizontally by means of a knob |42 until a minimum current reading is indicated on the meter |34. Obviously, this will be the point where the probe |4| intercepts the standing wave at its lowest power level. Then the probe is moved until it intercepts the standing wave at a point of maximum power; this Will be indicated by maximum current reading on the meter |34. The attenuating element |32 is then lowered into the pickup guide |23 by turning the thumb nut |33 introducing sufficient attenuation to reduce the value of the maximum current until the meter |34 indicates the minimum current rst noted. A calibrated gage |43 on the attenuator |26 will then indicate directly the standing wave ratio, or if it has been so calibrated, in decibels.

What is claimed is:

1. A radio frequency attenuator comprising a section of a Wave guide, coupling flanges integral with said wave guide section for coupling the wave guide section in a high frequency system,

one of said flanges having an annular groove formed in the face thereof to form a radio-frequency choke, said wave guide section having a longitudinal slot in one of the sides thereof, a resistive attenuating element movable into said wave guide section through said slot without making electrical contact with said Wave guide section, manually adjustable means for moving said attenuating element into and retracting it from said wave guide section, and calibrated indicating means for indicating the depth of the penetration of the attenuating element in the wave guide section, said attenuating element being rectangular in shape and having a length of an integral number of half-wave lengths of the wave to be attenuated.

2. An ultra-high frequency attenuator comprising a section of hollow wave guide having connection means integral therewith for coupling the section into an ultra-high frequency system, said connection means having an annular hollow portion therein to provide an ultra-high frequency choke for preventing energy loss at the coupling, an adjustable attenuating means movable into and out of the section through a slot therein, said attenuating means extending 1ongitudinally a half-wave length or an integral multiple thereof and being of uniform resistive character per unit of length, and an indicating means secured to the attenuating means for indicating the position thereof as a measurement of the attenuation introduced into the ultra-high frequency system.

HARRY R. LARSON.

REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,151,157 Scheikunoii.' Mar. 21, 1939 2,197,123 King Apr. 16, 1940 2,337,934 Scheldorf Dec. 28, 1943 2,358,462 Mahren Sept. 19, 1944 2,376,785 Krasik May 22,1945 2,395,851 Cork Mar. 5, 1946 2,407,267 Ginzton Sept. 10, 1946 2,441,598 Robertson May 18, 1948 

